To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Condition: The coin has a condition of 85 (MS+/EF-), indicating that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- It has been auctioned 8 times in the past, of which it has sold 6 times.
- The average hammer price in previous auctions is 2750 euros.
- Previous hammer prices for coins with a similar condition (85) have varied, but they have sold for prices ranging between 2400 and 3600 euros.

3. Starting Price and Estimates:
- The current starting price is 2500 euros, which is consistent with previous starting prices.
- Estimated prices in previous auctions have been higher than the current starting price, suggesting there is potential for a higher hammer price.

4. Price Trends:
- In previous auctions, coins with a condition of 85 have had hammer prices ranging from 2400 to 3600 euros, with an average of 2750 euros.

Based on this data, an estimated maximum purchase price could be around 3600 euros, considering that it is a price that has been reached in the past for coins of similar condition and that interest in the coin could lead to competitive bidding. However, since the average hammer price is 2750 euros, a more conservative range could be between 2750 and 3200 euros, depending on the demand at the auction.

Therefore, a reasonable maximum purchase price could be 3600 euros.